1. Field of the Invention
The present invention relates to an aqueous primer composition that can form high-brightness and high-chroma paint colors on plastic molded articles such as automobile bumpers, and that is capable of forming primer coating films with sufficient conductivity and excellent water resistance and humidity resistance, as well as to a coating method using the composition.
2. Description of the Related Art
Plastic materials used in automobile bumpers and the like normally have volume resistivity of about 1010 Ω/cm2 or greater, and because it is difficult to accomplish direct coating of such plastic molded articles by electrostatic coating methods which generally offer superior coating efficiency, it is common in the prior art to first apply a conductive primer coating onto the material surface and then carry out electrostatic coating of the top coat which is to provide the color (for example, see Japanese Unexamined Patent Publication HEI No. 6-165966).
Plastic materials such as mentioned above generally have dark colors such as black, and when they are to be coated with tints or low-opacity top coats, their conductive primer coats must be highly opaque or brightly colored. Several electrically conductive coatings have been proposed that employ high conductive fillers with relatively high brightness (for example, see Japanese Unexamined Patent Publication HEI No. 9-12314, Japanese Unexamined Patent Publication HEI No. 10-53417, Japanese Unexamined Patent Publication No. 2006-219521). Antimony-doped conductive fillers, which exhibit relatively high brightness and satisfactory conductivity among such fillers, have the drawback of toxicity, while needle-like conductive materials are avoided from the viewpoint of safety and hygiene.
When water-based coatings are used among the aforementioned electrically conductive coatings, with a view toward safety and hygiene and environmental conservation, it is necessary to employ large amounts of emulsifier to form aqueous dispersions of chlorinated polyolefins or the like, that are used as vehicle components, while the primers containing such components often yield coated films with inadequate water resistance and humidity resistance, with particular problems of reduced water resistance, humidity resistance and gasohol resistance being presented when thick films are formed by low-temperature baking at below 90° C.